Laboratory performance and commissioning status of the SALT NIR integral field spectrograph

Abstract

Washburn Astronomical Laboratories of the University of Wisconsin-Madison Astronomy Department has developed a near infrared (NIR) integral field spectrograph for the 11-meter Southern African Large Telescope (SALT). This instrument will be the first to extend SALT’s capabilities into the NIR, providing medium resolution spectroscopy at R = 2000-5000 over the wavelength range of 0.8 to 1.7 microns. The spectrograph is cooled to -40 C in an enclosure beneath the telescope, with the cryogenic dewar inside this enclosure operating at 120 K via a separate closed cycle cooler. The spectrograph uses volume phase holographic gratings with an articulated camera for setup versatility. The integral field unit (IFU) is an elongated hexagonal bundle of 212 fibers, each of which subtends 1.3 arcsec on the sky, matching the median site seeing. The IFU has on-sky dimensions of 29 x 18 arcsec, ideally suited for studies of nearby galaxies. A separate 38-fiber sky bundle can be adjusted to distances ranging 48.6 to 159.2 arcsec from the object IFU with a gimbaled jaw that maintains telecentricity and equal field angles for the object and sky bundles. Sky fibers are interleaved with object fibers along the 8-arcmin long spectrograph slit. The instrument has been fully tested in the laboratory and began installation and commissioning on SALT in May 2022. In this paper we present design highlights of the as-built instrument and IFU, optical alignment procedures, performance results from laboratory characterization, and the current status of installation and commissioning on the telescope.